Communication systems have traditionally been built around an arrangement in which Access and Processing Modules (APMs) occupy the same physical shelf. APMs are typically input/output cards with some processing capabilities. These APMs are linked together by a high speed, common parallel bus (shared bus) on a backplane having a high number of lines.
The backplane generally accepts APMs via slot connectors in order to connect them to other parts of the system. Motherboards on modern personal computers can be considered to be a general equivalent to backplanes in communication systems. Backplanes also typically distribute power to each module connected thereto.
Backplanes are often described as being either active or passive. Active backplanes contain, in addition to the sockets or slot connectors, logical circuitry that performs computing functions. In contrast, passive backplanes contain almost no computing circuitry. In high reliability environments, passive backplanes are generally preferred to active backplanes.
Modern day high performance systems have dictated steadily increasing performance requirements. As such, backplane arrangements have evolved so as to incorporate a switching core linking the APMs. In such an arrangement, dedicated, very high-speed serial links are used between the APMs and the switching core.
The functions of the switching core linking the APMs are typically put on one or more Switch Modules (SWMs). Implementations employing SWMs have made it easier to repair faulty components and to upgrade to newer and better components. However, the number of serial connections out of each APM to each SWM in such an implementation has skyrocketed.
At the same time, to push densities further, there is a recent tendency away from the use of a single sided shelf employing a standard backplane. More and more dual sided shelves are being used which employ a midplane organization rather the single-sided backplane. Although discussions in this document will concentrate on midplanes, it is to be understood that such discussions apply equally to a single sided shelf with a conventional backplane.
With these increasing densities and the increasing number of serial connections in high performance systems, it would be ideal if one could simply increase the size of the midplane indefinitely. However, there are some constraints that prevent this indefinite increase of midplane size. Firstly, at very high speeds (2.5 Gbps and more), differential electrical serial links can only travel a limited distance before attenuation increases error rates beyond control and signal integrity is seriously impacted. As such, there is a physical limit on the length of the serial link imposed by electrical transmission characteristics.
Secondly, manufacturing economics make it very difficult to increase the size of both APMs and SWMs on one hand, and the midplane itself on the other hand. When a certain size is reached in any of these cases, manufacturing yields decrease and costs increase, making it unreasonable to pursue an increased physical size.
Additionally, the APMs and SWMs connected to a midplane have a large number of very fine pitch active devices that need soldering. The high precision soldering equipment used for active APM and SWM assembly can only accommodate boards of limited maximum board dimension when compared with the capacity of equipment used to press fit connectors onto passive backplanes. Consequently, practical board dimensions are smaller for active circuit packs than for passive backplanes. Also, SWMs typically require many more midplane connections than APMs. As such, it is somewhat difficult to accommodate both types of modules in a single midplane. Furthermore, in a recent design (year 2000), economic limits were approached on a 36″ tall by 23″ wide midplane, and 22″ tall APM and SWM.
Therefore, there is a need to provide an arrangement that will overcome the drawbacks of the prior art and permit the connection of APMs and SWMs in such a way that the higher performance requirements of modern high-speed communication systems may be achieved.